The threat to aquatic habitats has grown steadily over the course of the twentieth century and continues today for a variety of reasons, most of which involve human intervention via overexploitation, introduced species, habitat alterations, pollution, and international trade. However, until recently, researchers did not fathom the scope of the problem among marine species because they assumed that broad distributions, the method of reproduction (pelagic dispersal), and the vastness of the marine environment might create a buffer to threats such as overexploitation and ecological decline. Unfortunately, there are worrying signs, such as collapses in many of the world’s fisheries and drastic declines in many large, mobile species (e.g. tunas). Additionally, researchers are finding that some species live quite long and have low reproduction and growth rates, meaning that removal of larger individuals can have significant impacts on populations. Another trade-related threat is excessive removal of exotic reef species using harsh chemicals, such as cyanide, for the aquarium trade.
Freshwater groups, however, account for the vast majority of actual extinctions in ray-finned fishes. The most significant threats are to families with restricted distribution (i.e. endemic) because localized threats can easily eliminate all individuals of a species. Introduced species, such as Nile perch and mosquitofish (genus Gambusia), combined with pollution and habitat alteration have proven particularly disastrous for groups of endemic ray-finned fishes (i.e. cichlids and many cyprinids). At this point, approximately 90 species of ray-finned fishes are known to be extinct or only survive in aquaria, 279 are critically endangered or endangered, and another 506 are listed as vulnerable or near threatened. Families of particular concern (in descending order) are cyprinids , cichlids , silversides , pupfishes , and especially sturgeons and paddlefishes since every member in the latter two families are threatened.